Hello
Thank you for your question and reaching out. I can understand you are having issues related to VM stuck.
- I would suggest you to first try power-cycle your Hyper-V server to clear this stuck merging or clear VSS state.
- If you have any Antivirus or security software please try to uninstall or disable for sometime.
- As you had already tried to merged for one of your smaller VM's .vhdx and .avdx files with unsuccessful result due corruption or CRC integrity mismatch, I would highly recommended to try to recover it from your Veaam backup from good copy.
- To Save your time as its seems due to Veaam backup application ,I will also advise you to engage Veaam
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